The cessna camera hole was a option from S/N 33701463 through 33701874, it's shown in the IPC figure 37A. it's right behind the copilot's seat rails, I'll try to attach a pic of what it looks like from inside. I dont know of any STC'd camera provisions for the 337, however depending on the type of shooting you are planning on do there are several strut mount platforms that may be adaptable to your uses (
http://www.lastrefuge.co.uk/), and it looks like a company has put together a very nice package for the singles (172,182) over at (
http://www.airbornescientific.com/content/cessnacam).
hope this helps
cC